This invention relates generally to mode coupled tuning fork type quartz crystal vibrators, and particularly to electrodes disposed in the region of the top of the tuning fork arms for adjusting the temperature-frequency characteristics of the vibrator.
Conventional electronic wristwatches generally utilize a tuning fork type quartz crystal vibrator which oscillates in the flexural mode. Such vibrators generally are cut from a +5.degree. X-cut. These tuning fork type quartz crystal vibrators are advantageous with respect to accuracy, because the frequency-temperature characteristic is represented by a parabola at room temperature and the power consumption is relatively low due to its low frequency. However, even when this type vibrator is utilized in a conventional electronic wristwatch, the monthly rate of time accuracy is about 20 seconds, at best. Thus, in order to improve further the time accuracy and to construct an electronic wristwatch having an extended useful life, a mode coupled tuning fork type quartz crystal vibrator having a frequency-temperature characteristic in the shape of a cubic curve at low frequencies in room temperature has been proposed.
Mode coupled tuning fork type quartz crystal vibrators are described in detail in Japanese Patent Application Nos. 53-23,903, 53-149,499 and 53-149,500. These latter two Japanese patent applications correspond to U.S. application Ser. No. 42,732, filed on May 29, 1979 and assigned to the same assignee as this application. In the mode coupled tuning fork type quartz crystal vibrators described in these earlier patent applications and provided herein, a first overtone at the flexural mode is utilized as a main vibration and a fundamental vibration in the torsional mode is utilized as a sub-vibration. The resonant frequency of the first overtone in the flexural mode (F.sub.1) is denoted by f.sub.F and the resonant frequency of the fundamental vibration in the torsional mode (T.sub.0) is denoted by f.sub.T. The difference between f.sub.F and f.sub.T is defined as .DELTA.f=f.sub.F -f.sub.T which determines the frequency-temperature characteristics of the mode coupled tuning fork quartz crystal vibrator. When .DELTA.f is a specified value, .DELTA.f.sub.0, the frequency-temperature characteristic in the form of a cubic curve may be obtained at ambient temperatures.
When producing mode coupled tuning fork type quartz crystal vibrators problems arise as the frequency f.sub.F and f.sub.T vary due to inevitable variation in dimensions of the external form of the vibrators. Consequently, .DELTA.f varies among the vibrators as does the frequency-temperature characteristic of each sample. It is also a problem that the resonant frequency of the main vibration is not the specified value in the same manner as in the case of a vibrator oscillating in a single mode. Thus, in order to be able to mass produce mode coupled tuning fork type quartz crystal vibrators, it is essential to be able to adjust the frequency-temperature characteristics and the resonant frequency of the main vibration. Accordingly, it is desirable to overcome the shortcomings in being able to mass produce mode coupled tuning fork type quartz crystal vibrators.